A vacuum heat-insulating material has a configuration in which a core material is tightly closed and decompressed and sealed on the inside of an outer cover material (outer package material) having gas barrier properties. As the outer cover material, generally, a laminated film on which functional layers, such as a thermal welding layer, a front surface protecting layer, and a gas barrier layer, are laminated is used.
The vacuum heat-insulating material is widely used in consumer products, such as electric appliances or housing materials, but in recent years, the use thereof has been further investigated in industrial products. Examples of the industrial product include a ship, such as a gas tanker, a heat-insulating container for holding low-temperature fluid, such as an LNG (liquefied natural gas) tank, and a vehicle (for example, for maintaining a temperature of a vehicle body, an engine, a transmission, or battery).
When the outer cover material is damaged, the gas barrier properties of the vacuum heat-insulating material are lost or deteriorate, and thus, it becomes difficult to maintain a substantially vacuum state (tightly closed and decompressed state) on the inside of the vacuum heat-insulating material. Here, from the related art, a technology of improving strength or durability of the outer cover material is suggested. For example, in PTL 1, a vacuum heat-insulating material in which a general refrigerator, a freezing machine, and a cooling and heating machine are assumed and pin-hole resistance of the outer cover material is improved, is disclosed. In the vacuum heat-insulating material, as the thermal welding layer of the outer cover material, a film in which tensile elongations in the longitudinal direction and in the lateral direction are respectively equal to or greater than 400% and the tensile elongation in the longitudinal direction with respect to the tensile elongation in the lateral direction is equal to or less than two times, is used.
However, in the industrial product, compared to the consumer product, there is a tendency that requiring properties of the vacuum heat-insulating material are strict. For example, in the ship, such as the above-described gas tanker, since the low-temperature fluid having a temperature which is substantially lower than a normal temperature is held for a long period of time, the vacuum heat-insulating material is used for a long period of time under the low-temperature environment. In addition, when performing maintenance of the ship, since there is a case of being exposed to a temperature which is higher than the normal temperature, the vacuum heat-insulating material is used not only under the low-temperature environment but also under the environment where an extremely large temperature difference is generated (for convenience, high-temperature difference environment). Furthermore, in the ship, compared to the consumer product, the use (for example, several tens of years) for a longer period of time is assumed, and thus, in the vacuum heat-insulating material, reliability for a longer period of time is acquired.
Here, under the low-temperature environment, according to a change or the like in an outdoor temperature, expansion and contraction of the outer cover material is generated, and thus, in the outer cover material, repetitive expansion and contraction stress is generated. In addition, under the high-temperature difference environment, in the outer cover material, large thermal stress which follows the temperature difference is generated. In this manner, reliability of the vacuum heat-insulating material for a long period of time is realized, and further, long-term durability of the outer cover material is acquired.
However, in the related art, in the vacuum heat-insulating material, in order to make it possible to employ the vacuum heat-insulating material in the industrial product, at least one of the strength and the durability of the outer cover material is improved, and further optimization of reliability of the outer cover material is rarely investigated. Furthermore, in the consumer product, for example, in a dwelling wall, in order to make it possible to correspond to the use in an extremely cold place, there is also a case where properties which can stand an extremely low outdoor temperature are required. However, in the related art, as the properties required in the vacuum heat-insulating material, such a condition which is stricter than a normal condition is rarely assumed.